Recently electrooptic TE.revreaction.TM mode conversion has been demonstrated in Ti-diffused lithium niobate waveguides. (See, for example, the article by R. C. Alferness entitled "Efficient, Waveguide Electrooptic TE.revreaction.TM Mode Converter/Wavelength Filter" published among the Post deadline papers of the Optical Communication Conference, Amsterdam, Sept. 17-19, 1979, pp. 19.6-1 to 4.) Because of the large birefringence of lithium niobate, periodic coupling is required to achieve phasematching. This, in turn, makes the device highly wavelength selective. As a result, such a structure can be used as a wavelength filter. However, to extract the filtered (i.e., the mode converted) signal, a separate polarization selective element is required. Furthermore, the polarization sensing element must be of a high quality in order to satisfy the stringent requirements needed to avoid crosstalk in the system.
It is, accordingly, the broad object of this invention to effect wavelength filtering using mode conversion techniques which spatially separate the filtered signal from the rest of the signal, thereby avoiding the need of a separate polarization selective element.